There are many examples of displays mounted on the head, sunglasses, eyeglasses and the like. For example, Perera in U.S. Pat. No. 4,867,551 describes a display to be mounted on eyeglasses, the limitation of which is the high degree of occlusion of the user's field of view beyond the display, and the use of non-axial optics which introduces distortion. A high degree of occlusion is also present in an eyeglass mounted display offered for sale by Albacomp Computers Corp. Other companies, such as Liquid Image or Virtual Vision, provide displays that are suspended by a cable, gooseneck fixture or other mechanical support in front of one or both of the user's eyes. Similarly, displays from Reflection Technology have been mounted on eyewear in order to provide a computer display in a mobile fashion. This approach also highly limits the user's view of the surroundings.
Not only do such prior art displays obscure a large part of the user's visual field, but also the display obscures the user's face. The obscuration of the visual field is a limitation particularly in applications in which the user must be free to focus on the work at hand, without diverting attention to the display.
Generally, head-mounted and helmet-mounted display systems are based on miniature displays having a diagonal dimension of 4 cm or less. The display systems that use such miniature displays must provide a lens near the eye for magnification, and to make possible comfortable viewing at near distances. The lens and any other associated optics that must be placed near the eye are termed the "eyepiece." Most prior art head-mounted systems also place the display (for example a miniature liquid crystal flat panel display) near the eye as well, which requires both a support fixture for the eyepiece, and a conduit for electrical cables to the display. These components (wires, liquid crystal display, and any other required circuits) are placed within an opaque housing near the eye. Consequently, such systems block a portion of the user's visual field, and also obscure the user's face. Obscuration of the face reduces the desirability of using the device in social occasions.
A helmet-mounted display is also known in which the image is provided by a glass fixture suspended in front of the user's face. The glass fixture is mounted to the helmet and contains an eyepiece near the eye. The glass fixture serves as a mechanical support for the eyepiece as well as an image conduit for the display which is mounted to the helmet. The limitation of this system, however, is its size and weight, and its configuration as a helmet.